Shaykh al-Islaam Ibn Taymiyyah (rahimahullaah) has an explicit statement indicating that requesting the dead to supplicate to Allaah on their behalf (for fulfilment of certain needs) is the essence of shirk. In his book, Iqtidaa Siraat al-Mustaqeem (p. 406, tahqeeq Muhammad Hamid al-Fiqqee and p. 553, commentary of Ibn al-Uthaymeen) he said:

And the intent here is that the shirk through worship of the stars has occurred a great deal and likewise shirk through those in the graves, through supplicating to them, imploring them, seeking them (with aspiration) and what is like that. For when the Prophet (sallallaahu alayhi wasallam) has prohibited prayer which comprises sincere supplication to Allaah alone in the proximity of graves so that it does not lead to a type of shirk with their Lord then how (would it be) when there is found what (comprises) the type of shirk of aspiration (raghbah) towards them, regardless of whether he requested needs and removal of difficulties from them directly or he requested them that they request it from Allaah (for them)...

Note:

In this passage, Ibn Taymiyyah does not make any differentiation between a person saying for example, "O Fulaan, cure my illness, grant me a child, relieve of my calamity..." and the likes and between a person saying, "O Fulaan, call upon Allaah that he cures my illness, grants me a child, relieves me of my calamity..." and whilst the first is no doubt a greater form of shirk, it does not mean that second one is not shirk, rather it is also shirk, since there are numerous things going on in this scenario, from them is making du'a to other than Allaah (asking the dead to make du'a is in itself du'a), it is also seeking their intercession (shafaa'ah means to ask another to supplicate for you), it is also making them as intermediaries through whom needs are fulfilled.

Update (03/11/1434H - 08/09/2013CE): This article was updated where the word (عين) in the phrase (فكيف إذا وجد ما هو عين الشرك الشرك) was updated to (فكيف إذا وجد ما هو نوع الشرك الشرك). The confusion is that in some (printed) versions the word (عين) has been used whereas that which is most reliable (in manuscripts) and also printed verified versions of al-Iqtidaa, the word (نوع) has been used. However, as far as the meaning goes, then refer to this article, which elaborates further.